Method for detecting failures in trunk status identification circuitry

ABSTRACT

An arrangement for detecting failures in the trunk status identification circuitry which is used to determine the status of calls passing through a system. A scanpoint test is performed once during each scan cycle to determine the condition of the scanpoint circuits and the AC bus drivers which normally activate the scanpoints. The AC bus drivers also are tested for both set and reset, each time prior to activating the AC bus drivers during normal system operation.

United States Patent [1 1 Padgett We. 4, 1973 [54] METHOD FOR DETECTINGFAHLURESIN 3,713,104 1/1973 Stich 179/1752 C TRUNK STATUS IDENTIFICATIONCIRCUITRY Primary Examiner-William C. Cooper [75] Inventor: Richard A.Padgett, Lombard, 11]. Assistant Examiner Douglas W. Olms [73] Assignee:GTE Automatic Electric AttomqH'K' Munerhelm et Laboratoriesincorporated, Northlake, 111. [22] Filed: Oct. 2, 1972 [57] ABSTRACT 21APPL 293 977 An arrangement for detecting failures in the trunk statusidentification circuitry which is used to determine the status of callspassing through a system. A scan- [52] US. Cl l7/l75.2 R point test isperformed Once during each Scan cycle to [51] Illt. Cl. H04"! 3/26determine the condition of the scanpoint circuits and [58] Fleld ofSearch 179/1752 R, 175.2 C, the AC bus drivers which normally activatethe Scan 179/7 8 R points. The AC bus drivers also are tested for bothset and reset, each time prior to activating the AC bus [56] Referencescued drivers during normal system operation.

UNITED STATES PATENTS 2,866,008 15 Claims, 6 Drawing Figures CLEAR ALARM7 12/1958 Walsh 179/1752 R AC BUS ALL SET 60 RCVR FROM OR AC $ET7 MEMORRECEIVER I STATUS TEST RESET; r- 1 FRAME Eou/PPED (TFE) I CIRCUIT AC BUSRCVR ALL RESET NAND AND 1557- AC BUS I RcvR I 22 I 76 FAILED 40, s WMAINCKTS Wu F I? 75 R 72 I I NAND AND M, 1

AND

STROBE) TENS A INTERROGATE PULSE IuN/Ts K AC BUS I To I OTHER DRFIB/0055 f 59 GROUP I I ADDRESS 61 TCT I M68 1 I f W W DRIVER GlcDR/vERs l 2 661 57 I TOT L TRK CKTI TEsTc/ TEST TEST I9 AND M0057 ANDME) FROM 62 MEMORY I 54 R --I R R AND AND v I '65 e5 PATENTEDDEE 4:915

atwmeo SHEET 3 UP 6 Txll Tx/l Txl/ CLEAR ADVANCE RESET com/wow A MEMORY55.

T R ss CK 5 ADD E a WRITE Txl/ y Tx/Z RESET READ BLOCK NEXT w WRITE WORDTx9 8 Tx/O n9 WRITE SET BLOCK MEMORY Wm:-

T I STORE P05 LOA TT 553;; TE/vs DIG/TS 5; P052 UNITS RCVRS //v E0.DIG/T //v E0.

- P05 REG. Pas REG.

056005 TRK. Pas. N0.

Tx7 w UPDATE MEMORY ADDRESS STATUS a STATUS To no" STORE TRK.

STATUS IN STATUS RCVRS.

ACT/ VA TE GRE DR! VERS STORE STA TUS GRP REG.

DE CODE E0. GRP.

PATENTEDuEc 41915 3,777,080

60 ADVANCE TO START MEMORY DDR. I ADDR Txl6 Hi3 SET BLOCK LOAD READ a OSIN START WRITE 8 E62 R a RESET TEST P05. REGS.

OPERATE YES SET 5 flrigjg COMPLEMENT FOR OPEN TEST. MODE TEST 5 W] ES ORC 8E TEST YES Txo

SET ALL ENABLE smrus smru Tags RCVRS RCVRS UNITS W Txo T213 REQ. FORCEDSTORE RLS a 5E7 RESET am? No.

Arus F05 RCVRS & TF5 B/Ts ENABLE v TEST GATES DECODE FOR Q TFE EQU/PPEDBITS I PATENTEDHH: 4 I975 CLEAR ALARM 7 SHEET 6 OF 6 ALLSET so MEMOR 1/50R RECEIVER TEsT FRAME EOU/PPED (TFE) IRCUIT A0 a s RCV ALL RESET NANDAND TEST 509? I FAILED wMA/M F CKTS. 0R

NAND AND a AND 'STROBE) TEAM #1 v INTERROGATE PULSE TO M BUS OTHER mm"7' DIODES J GROUP ADDRESS T F 1 TEsT v I l TEST A "[35,? DRIVERS l 2 li I 66 J 67 M68 2 L J. 3 7 TRK. CKTI TESTV TEST TEST) Moos) MRE 3 [97 sr s I "LJS FROM 62 M MEMORY I14 R R R 7 AND U AND H n WNW.

METHOD FOR DETECTING FAILURES IN TRUNK STATUS IDENTIFICATION CIRCUITRYBACKGROUND OF THE INVENTION Field of the Invention This inventionrelates to a centralized automatic message accounting system. Moreparticularly, it relates to an arrangement for detecting failures in thetrunk status identification circuitry in such a system.

In the hereinafter disclosed centralized automatic message accountingsystem, trunk status identification circuitry is used to determine thestatus of calls passing through the system. A trunk scanner is connectedto the trunks by a highway extending from a billing unit to each trunk,and is the means of conveying the various states of the trunks to thebilling unit. Potentials on the highway leads indicate the status of thetrunks.

The trunk scanner upon reading a start test word from memory activatesthe status identification circuitry to set up a scanpoint test, byturning on an established number of test driver circuits that send aconstant test signal simultaneously to all billing unit scanpoint testcards in all the trunk frames, to determine if the contacts of the trunkcircuits are open or closed. If a contact is closed, a signal isreturned to a bus receiver to indicate the status of the contact.

The arrangement of the present invention provides a method for testingand detecting failures in the described trunk status indentificationcircuitry. The arrangement, furthermore, is such that the area where thefailure occurred can be identified and indicated.

Accordingly, it is an object of the present invention to provide animproved centralized automatic message accounting system.

More particularly, it is an object to provide an arrangement fordetecting failures in the trunk status identification circuitry used insuch a system.

A still further object is to provide such an arrangement wherein thearea within the status identification circuitry where the failureoccurred can be identified.

Still another object is to provide such an arrangement wherein certaintests are performed on alternate scan cycles so that the same testfunctions can be used for these tests without additional memory beingrequired.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of thenature and objects of the invention, reference should be had to thefollowing detailed description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a functional block diagram of the centralized automaticmessage accounting system;

FIG. 2 illustrates the trunk scanner memory layout for the statussection and the test sections thereof;

FIGS. 3-5 are a flow chart of the trunk scanner operations; and

FIG. 6 is a functional block diagram of the arrangement for detectingfailures in the status identification circuitry.

Similar reference characters refer to similar parts throughout theseveral views of the drawings.

DESCRIPTION OF THE INVENTION Referring now to the drawings, in FIG. 1the centralized automatic message accounting system is illustrated inblock diagram, and the functions of the principal equipment elements canbe generally described as follows. The trunks 10, which may be eithermultifrequency (MF) trunks or dial pulse (DP) trunks, pro vide aninterface between the originating office. the toll switching system, themarker 11, the switching network 12, and the billing unit 14. Theswitching network 12 consists of three stages of matrix switchingequipment between its inlets and outlets. A suitable distribution oflinks between matrices are provided to insure that every inlet has fullaccess to every outlet for any given size of the switching network. Thethree stages, which consist of A, B and C crosspoint matrices, areinterconnected by AB and BC links. The network provides a minimum ofinlets, up to a maximum of 2000 inlets and 80 outlets. Each inletextends into an A matrix and is defined by an inlet address. Each utletextends from a C matrix to a terminal and is defined by an outletaddress.

Each full size network is divided into a maximum of 25 trunk grids onthe inlet side of the network and a service grid with a maximum of 16arrays on the outlet side of the network. The trunk grids and servicegrid within the networks are interconnected by the BC link sets of 16links per set. Each MF trunk grid is provided for 80 inlets. Each DPtrunk grid is provided for 40 inlets. The service grid is provided for amaximum of 80 outlets. A BC link is defined as the interconnection of anoutlet of a B matrix in a trunk grid and an inlet of a C matrix in theservice grid.

The marker 11 is the electronic control for establishing paths throughthe electromechanical network. The marker constantly scans the trunksfor a call for service. When the marker 11 identifies a trunk with acall for service, it determines the trunk type, and establishes aphysical connection between the trunk and a proper receiver 16 in. theservice circuits 15.

The trunk identity and type, along with the receiver identity, aretemporarily stored in a marker buffer 17 in the call processor I8 whichinterfaces the marker 11 and the call processor l8.

When the call processor 18 has stored all of the information transmittedfrom a receiver, it signals the marker II that a particular trunkrequires a sender 19. The marker identifies an available sender,establishes a physical connection from the trunk to the sender, andinforms the call processor 18 of the trunk and sender identities.

The functions of the receivers 16 are to receive MF 2/6 tones or DPsignals representing the called number, and to convert them to anelectronic 2/5 output and present them to the call processor 18. Acalling number is received by MP 2/6 tones only. The receivers will alsoaccept commands from the call processor 18, and interface with the ONItrunks 20.

The function of the MF sender's are to accept commands from the callprocessor 18, convert them to MF 2/6 tones and send them to the tollswitch.

The call processor 18 provides call processing control and, in addition,provides temporary storage of the called and calling telephone numbers,the identity of the trunk which is being used to handle the call, andother necessary information. This information forms part of the initialentry for billing purposes in a multientry system. Once'this informationis passed to the billing unit 14, where a complete initial entry isformated, the call will be forwarded to the toll switch for routing.

The call processor 18 consists of the marker buffer 17 and a callprocessor controller 21. There are 77 call stores in the call processor18, each call store handling one call at a time. The call processor 18operates on the 77 call stores on a time-shared basis. Each call storehas a unique time slot, and the access time for all 77 call stores isequal to 39.4 MS, plus or minus 1 percent.

The marker buffer 17 is the electronic interface between the marker 11and the call processor controller 21. Its primary functions are toreceive from the marker 11 the identities of the trunk, receiver orsender, and the trunk type. This information is forwarded to theappropriate call store.

The operation of the call process controller revolves around the callstore. The call store is a section of memory allocated for theprocessing of a call, and the call process controller 21 operates on the77 call stores sequentially. Each call store has 8 rows and each rowconsists of 50 bits of information. The first and second rows arerepeated in rows 7 and 8, respectively. Each row consists of twophysical memory words of 26 bits per word. Twenty-five bits of each wordare used for storage of data, and the 26th bit is a parity bit.

The call processor controller 21 makes use of the information stored inthe call store to control the progress of the call. It performs digitaccumulation and the sequencing of digits to be sent. It performs fourthdigit O/l blocking on a 6 or 10 digit call. It interfaces with thereceivers 16, the senders 19, the code processor 22, the billing unit14, and the marker buffer 17 to control the call.

The main purpose of the code processor 22 is to analyze call destinationcodes in order to perform screening, prefixing and code conversionoperations of a nature which are originating point dependent. This codeprocessing is peculiar to the needs of direct distance dialing (DDD)originating traffic and is not concerned with trunk selection andalternate routing, which are regular translation functions of theassociated toll switching machine. The code processor 22 is accessedonly by the call processor 18 on a demand basis.

The billing unit 14 receives and organizes the call billing data, andtranscribes it onto magnetic tape. A multi-entry tape format is used,and data is entered into tape via a tape transport operating in acontinuous recording mode. After the calling and called directornumbers, trunk identity, and class of service information is checked andplaced in storage, the billing unit 14 is accessed by the call processcontroller 21. At this time, the call record information is transmittedinto the billing unit 14 where it is formated and subsequently recordedon magnetic tape. The initial entry will include the time. Additionalentries to the billing unit 14 contain answer and disconnectinformation.

The trunk scanner 25 is the means of conveying the various states of thetrunks to the billing unit 14. The trunk scanner 25 is connected to thetrunks by a highway extending from the billing unit 14 to each trunk.Potentials on the highway leads will indicate states in the trunks.

Each distinct entry (initial, answer, disconnect) will contain a uniqueentry identity code as an aid to the electronic data processing (EDP)equipment in consolidating the multi-entry call records into tollbilling statements. The billing unit 14 will provide the correct entryidentifier code. The magnetic tape unit 26 is comprised of the magnetictape transport and the drive, storage and control electronics requiredto read and write data from and to the 9 channel billing tape. The readfunction will allow the tape unit to be used to update the memory.

The recorder operates in the continuous mode at a speed of five inchesper second, and a packing density of 800 bits per inch. Billing data isrecorded in a multientry format using a 9 bit EBCDIC character (extendedbinary coded decimal interchange code). The memory subsystem 30 servesas the temporary storage of the call record, as the permanent storage ofthe code tables for the code processor 18, and as the alterable storageof the trunk status used by the trunk scanner 25.

The core memory 31 is composed of ferrite cores as the storage elements,and electronic circuits are used to energize and determine the status ofthe cores. The core memory 31 is of the random access, destructivereadout type, 26 bits per word with 16 K words.

For storage, data is presented to the core memory data registers by thedata selector 32. The address generator 33 provides the address or corestorage locations which activate the proper read/write circuitsrepresenting one word. The proper clear/write command allows the dataselected by the data selector 32 to be transferred to the core storageregisters for storage into the addressed core location.

For readout, the address generator 33 provides the address or corestorage location of the word which is to be read out of memory. Theproper read/restore command allows the data contained in the word beingread out, to be presented to the read buffer 34. With a read/restorecommand, the data being read out is also returned to core memory forstorage at its previous location.

The method of operation of a typical call in the system, assuming theincoming call is via an MF trunk can be described as follows. When atrunk circuit 10 recognizes the seizure from the originating office, itwill provide an off-hook to the originating office and initiate acall-for-service to the marker 11. The marker 11 will check theequipment group and position scanners to identify the trunk that isrequesting service. Identification will result in an assignment ofaunique four digit 2/5 coded equipment identity number. Through atrunk-type determination, the marker 11 determines the type of receiver16 required and a receiver/sender scanner hunts for an idle receiver 16.Having uniquely identified the trunk and receiver, the marker 11 makesthe connection through the three-stage matrix switching network 12 andrequests the marker buffer 17 for service.

The call-for-service by the marker 11 is recognized by the marker buffer17 and the equipment and receiver identities are loaded into a receiverregister of the marker buffer 17. The marker buffer 17 now scans thememory for an idle call store to be allocated for processing the call,under control of the call process controller 21. Detection of an idlecall store will cause the equipment and receiver identities to be dumpedinto the call store. At this time, the call process controller 21 willinstruct the receiver 16 to remove delay dial and the system is nowready to receive digits.

Upon receipt of a digit, the receiver 16 decodes that digit into 2/5code and times the duration of digit presentation by the calling end.Once it is ascertained that the digit is valid, it is presented to thecall processor 18 for a duration of no less than 50 milliseconds ofdigit and 50 milliseconds of interdigital pause for storage in thecalled store. After receipt of ST, the call processor controller 21 willcommand the receiver 16 to instruct the trunk circuit to return anoff-hook to the calling office, and it will request the code processor18.

The code processor 18 utilizes the called number to check for EASblocking and other functions. Upon completion of the analysis, the codeprocessor 18 will send to the call processor controller 21 informationto route the call to an announcement or tone trunk, at up to four prefixdigits if required, or provide delete information pertinent to thecalled number. If the call processor controller 21 determined that thecall is an AN! call, it will receive, accumulate and store the callingnumber in the same manner as was done with the called number. After thecall process controller 21 receives ST, it will request the billing unit14 for storage of an initial entry in the billing unit memory. It willalso command the receiver 16 to drop the trunk to receiver connection.The call processor controller 21 now initiates a request to the markerll via the marker buffer 17 for a trunk to sender connection. Once themarker 11 has made the connection and has transferred the identities tothe marker buffer 17, the marker buffer will dump this information intothe appropriate call store. The call processor controller 21 nowinterrogates the sender 19 for information that delay dial has beenremoved by the routing switch (crosspoint tandem or similar). Uponreceipt of this information the call processor controller 2 1 willinitiate the sending of digits including KP and ST. The call processcontroller 21 will control the duration of tones and interdigital pause.After sending of ST", the call processor 18 will await the receipt ofthe matrix release signal from the sender l9. Receipt of this signalwill indicate that the call has been dropped. At this time, the senderand call store are returned to idle, ready to process a new call.

The initial entry information when dumped from the call store isorganized into the proper format and stored in the billing unit memory.Eventually, the call answer and disconnect entries will also be storedin the billing unit memory. The initial entry will consist ofapproximately 40 characters and trunk scanner 25 entries for answer ordisconnect contain approximately characters. These entries will betemporarily stored in the billing unit memory until a sufficient numberhave been accumulated to comprise one data block of 1370 characters.Once the billing unit memory is filled, the magnetic tape unit 26 iscalled and the contents of the billing unit memory is recorded onto themagnetic tape.

The final result of actions taken by the system on a valid call will bea permanent record of billing information stored on magnetic tape inmulti-entry format consisting of initial, answer, and disconnect orforced disconnect entries.

Answer timing, force disconnect timing and other timing functions suchas, for example, a grace period timing interval on answer, in thepresent system; are provided by the trunk timers. These trunk timers arememory timers, and an individual timer is provided for each trunk in atrunk scanner memory which, as can be best seen in FIG. 2, illustratesthe memory layout, comprises a status section and a test section.

The status section contains 1 :word per ticketed trunk. Each wordcontains status, instruction, timing and sequence information. Thestatus section also provides one word per trunk group which contains theequipmemt group number, and an equipment position tens word thatidentifies the frame. A fully equipped status section requires 2761words of memory representing 2000 trunks spread over 60 groups plus astatus section start" word. As each status word is read from memory, itis stored in a trunk scanner read buffer (not shown). The instruction isread by a scanner control to identify the contents of the word. Thescanner control logic acts upon the timing, sequence and statusinformation, and returns the updated word to the trunk scanner memoryand it is written into it for use during the next scanner cycle.

The test section contains a maximum of 83 words: a start word, a lastprogrammed word, 18 delay words, two driver test words, one end-testword and one word for each equipment group. The start test word causes ascan point test to begin. The delay words allow time for scan pointfilters to charge before the trunk groups are scanned, with the delaywords containing only instructional data. The equipment group wordscontain a two digit equipment group identity and five trunk frameequipped bits. The trunk frame equipped bits (one per frame) indicateswhether or not a frame exists in the position identified by its assignedbit. The delay words following the equipment group allow the scan pointfilters to recharge before the status section of memory is accessedagain for normal scanning. The Last Program word inhibits read and writein the trunk scanner memory until a trunk scanner address generator hasadvanced through enough addresses to equal the scanner cycle time. Whenthe cycle time expires. the trunk scanner address generator returns tothe start of the status section of memory and normal scanningrecommences.

The trunk scanner memory and the trunk scanner read buffer are not partof the trunk scanner 25, however, the operation thereof is controlled bya scanner control which forms a part of the trunk scanner 25 of thebilling unit 14-. The trunk scanner 25 maintains an updated record ofthe status of each ticketed trunk, determines from this status when abilling entry is required, and specifies the type of entry to berecorded. The entry includes the time it was initiated and theidentification of its associated trunk.

Scanning is performed sequentially, by organizing the memory in such amanner that when each word is addressed, the trunk assigned to thataddress is scanned. This causes scanning to progress in step with thetrunk scanner address generator. During the address advance interval,the next scanner word is addressed and, during the read interval, theword is read from memory and stored in the trunk scanner read buffer. Atthis point, the trunk scanner 25 determines the operations to beperformed by analyzing the word instruction.

Referring now to FIGS. 3, 4 and 5, which are flow charts of the trunkscanner operations, the operation of the latter as well as the trunktimer can be described.

As indicated above, scanning is performed sequentially. If all trunks inall groups are scanned in numerical sequence beginning with trunk 0000,scanning would proceed in the following manner:

Step I. Trunk 0000 located in frame 00 (lineup 0,

column 0) in the top file, leftmost card position would be scannedfirst.

Step 2. All trunks located in frame and the leftmost card position wouldbe scanned next from the top file to the bottom.

Step 3. Scanning advances to frame 01 (lineup 0, column l) and proceedsas in Step 2.

Step 4. Scanning proceeds as in Step 3 until frame 04 has been scanned.

Step 5. The scanner returns to frame 00 and Step 2 is repeated for thenext to leftmost card position.

Step 6. The sequence just described continues until all ten cardpositions in all five columns have been examined.

Step 7. The entire process is repeated in lineups 1 through 5.

When a memory word instruction identifies a trunk group word, the statusreceivers are cleared to prepare for scanning the trunks specified inthe group word. The trunk group digits stored in the trunk scanner readbuffer (TSRB) are transferred into the equipment group register.

After the trunk group number is decoded, it is transformed into binarycode decimals (BCD), processed through a l-out-of-N check circuit, andapplied to the AC bus drivers (ACBD). The drivers activate the scanpoint circuits via the group leads and the trunk status is returned tothe receivers.

A group address applied to the drivers causes the status of all trunksin one lineup and one card position and all columns to be returned tothe receivers. The group tens digit specifies the trunk frame lineup andthe group units digit identifies the card slot.

When a status word is read from memory, it sets the previous count of atrunk timer (TT) into the trunk timer.

If the trunk is equipped and the forced disconnct sequence equals 2(FDS=2), a request to force release the trunk is transmitted to themarker 11. If FDS does not equal 2, the present condition of theticketing contacts in the trunk is tested. If the instruction indicatesthat the trunk is in an updated condition (the trunks associated memoryword was reprogrammed) it is tested for idle. If the trunk is idle, itsinstruction is changed to denote that it is ready for new calls. If thetrunk is not idle, no action is taken and the trunk scanner 25 proceedsto the next trunk.

If the trunk is not in the updated condition and FDS=3, the trunk istested for idle. If the trunk is idle, FDS is set to 0 and TT is reset.

it FDS does not equal 3 and a match exists between the present contactstatus and the previous contact status stored in memory (bits 5 and 6)the FDS memory bits are inspected for a count equal to 1. If FDS=l, TTis reset and the memory contact status is updated. If FDS does not equal1, TT is not reset.

During any analysis of a trunk status, a change in the contactconfiguration of a trunk is not considered valid until it has beenexamined twice.

One bit (SFT) is provided in each memory status word to indicate whetheror not a change in status of the trunk was detected during the previousscan cycle.

When a change in status is detected, SPT is set to I. If SFT=1 on thenext cycle, the status is analyzed and SFT is set to 0.

If a mismatch exists between the present contact condition and thatpreviously stored in memory, the status has changed and a detailedexamination of the status is started.

If CT=1 the trunk is busy and so the previous condition of the contactis inspected. If the trunk previously was idle, CM=O. Before continuingthe analysis, it must be determined if this is the first indication ofchange in the trunk status by examining the second look" bit (SFT). lfSFT=O, it is set to equal 1, and the analysis of this trunk status isdiscontinued until the next scanner cycle. If SFT=l, the memory statusis updated and SP1" is set to equal 0.

If CT=l, the trunk is cut through and CM is inspected to determine ifthe memory status was updated. If CM=l, the GT contact status mustdiffer from GM since it was already determined that a mismatch exists.lf GT=0, answer has not occurred. If GT=l, and this condition existedduring the previous scan cycle, SFT=l also. If these conditions are trueand FDS does not equal 1, TT is advanced and answer timing begins. Ifthese conditions persist for 8 scanner cycles (approximately onesecond), answer is confirmed and an entry will be stored in the trunkscanner formater (TSF). If answer is aborted (possibly hookswitchfumble) before the one second answer time (time is adjustable) expires,TT remains at its last count. When the answer condition returns, answertiming continues from the last 'IT count. Thus, answer timing iscumulative.

After an answer entry is stored, which includes the TT count, TT isreset, SFT is set to 0, and the new contact status is written intomemory.

If a mismatch exists and CT=0, the previous state of this contact isinspected by examining bit 5 in the trunk scanner read buffer (TSRB). lfCM=l, the state of the terminating end of the trunk is tested. If GT=l,then the condition of the trunk has just changed from answer todisconnect. If this condition existed during the previous scan cycle,SFT=1 and a disconnect entry is stored in the TSF.

After the disconnect entry is stored, which includes the TI count, TT isreset, FDS and SFT are set to 0, and the new status is written intomemory.

Ifa mismatch exists and the originating end of a trunk is not released,both CT and CM equals 1. If GT=O after the previous scan cycle, FDS istested. If this change just occurred, FDS does not equal 1. Since FDSdoes not equal 1, it will be set equal to l and T1" will reset. FDS=1indicates that forced disconnect timing is in progress.

While the conditions just described exist, i.e., mismatch, CR=1, CM=l,GT=0 and FDS=1, TT will advance 1 count during each scanner cycle, ifone half second has elapsed since the last scan cycle. TT will continueto advance until it reaches a count of 20 (approximately 10 seconds)when a forced disconnect entry will be stored in the TSF.

When the entry is stored, FDS is set at 2 indicating that the trunk isto be force released. After the entry is stored, which includes the TTcount, TT is reset, SFl' is set to 0, and the new status is written intomemory.

After the status and test sections of the memory have been accessed, theLast Program word is read from memory and stored in the trunk scannerread buffer. This word causes read/write in the trunk scanner portion ofmemory to be inhibited and deactivates the scan point test. The trunkscanner address generator will continue to advance, however, untilsufficient words have been addressed to account for one scan cycle. Whena predetermined address, the Last Address, is reached, block read/writeis removed and the address generator returns to the Start Address (FirstProgram Word) of the scanner memory.

As indicated above, when a memory word instruction identifies a trunkgroup word, the status receivers are cleared to prepare for scanning thetrunks specified in the group word. The trunk group word, afterprocessing, is applied to the AC bus drivers (ACBD) which activate thescanpoint circuits via the group leads, and the trunk status is returnedto the AC bus receivers. This occurs while the status section of thetrunk scanner memory is being-scanned.

A scanpoint test is performed once during each scan cycle to determinethe condition of the scanpoint circuits and the AC bus drivers. Thisscanpoint test consists of two tests, one of which is an open test"during which the common ground connection to the trunk ticketingcontacts 60 (FIG. 6) is open. The open test makes all contacts appearopen whether they are open or not. The second test is a close test thatmakes all ticketing contacts 60 appear closed by grounding the diodes611 connected to each scanpoint. During this test, the common groundconnection to the contacts is left open as in the open test. The closetest and open test are performed during alternate scan cycles. The ACbus driver test is performed during the close test cycle, all as morefully described below.

The scanpoint test begins when the test section of the trunk scannermemory is accessed, and the start test word (FIG. 2) is read out. Thestart test word activates various switches on all scanpoint cards(notshown) by changing the test to the alternate mode, open or close.Actual testing is delayed, however, by first accessing the nine blankwords following the start test word to allow the integrating circuits ineach scanpoint to charge or discharge.

By performing the close tests and the open tests on alternate scancycles, the same test functions can be used for both tests withoutadditional memory being required. The manner in which these tests arealternated is as follows. If the flip-flops MRE, TEST MODE, and TEST(FIG. 6) are reset, the flip-flop TEST will set when the start test word(instruction I9) is read from the trunk scanner memory. When theflip-flop TEST sets, the AND gate 62 is enabled by the coincidence ofthe SET output from the flip-flop TEST and the RESET output from theflip-flop MRE to, in turn, cause the flip-flop TEST MODE to set anddisable both inputs to the flip-flop MRE via the AND gates 64 and 65.The flip flop TEST MODE when set specifies that the closedtest is to beperformed.

When the end test word (instruction I14) is read, the flip-flop TEST isreset and the flip-flop MRE is caused to set, with the AND gate 64 beingenabled by the coincidence of the RESET output from the flip-flop TESTand the SET output from the flip-flop TEST MODE.

On the next scan cycle, the start test word causes the flip-flop TEST toagain set and, in turn, cause the flipflop TEST MODE to reset, by meansof the coincidence of the SET input from the flip-flop TEST and the. SEToutput from the flip-flop MRE enabling the AND gate 63. The flip-flopTEST MODE, when reset, indicates that the open test" is to be performed.

When the end test word is read again, the flip-flop TEST is reset to, inturn, cause the flip-flop MRE to reset. On'the next scan cycle, theclose test" will be repeated, so that the open test" and close testoccur on alternate scan cycles.

When the start test word is read and the flip-flop TEST is set, the SEToutput also provides a TEST signal input to the test driver logic 66.The test driver logic 66, in turn, energizes 12 test drivers 67 totransmit a constant signal TOT simultaneously to all of the electroniccircuits MGSZ on all of the scanpoint cards. The signal TOT istransmitted during the time that the scanpoint test is conducted, inorder to de-energize the electronic circuits MGSZ to remove ground fromall of the trunk circuit contacts 60. During the close test", the SEToutput from the flip-flop TEST MODE is coupled as a CT signal to thetest driver logic 66, and the latter energizes the test drivers 67 totransmit a signal TCT to the electronic circuits MGSl. The electroniccircuits MGSl simulate a closure of the trunk circuit contacts 60, bygrounding the inputs to the scanpoints via the diodes 61. During theopen test, the RESET output of the flip-flop TEST MODE is coupled as anOT signal to the test driver logic 66 to energize the test drivers 67such that the signal TCT de-energizes the electronic circuits MGSI. Whenthe electronic circuits MGSl are de-energized, ground is removed fromthe scanpoints, thus simulating an open trunk circuit contact 60. Duringa no test period, the electronic circuits MGSl are deenergized and theelectronic circuits MGSZ are energized. If the latter are notreenergized after the scanpoint test is completed, an alarm sounds.

When the scanpoint test has been established, as described above, thefirst group word (instruction I13) is read from memory (FIG. 2) and theequipment group number and the TFE (trunk frame equipped) bits arestored in the equipment group register. The equipment group address(tens and units) then are coupled to the AC bus drivers ACBD to energizethem to provide an interrogate pulse'to the scanpoints, when the strobepulse is received coincicent therewith. The strobe pulse is a 4 u-sec.clock pulse gated to the AC bus drivers ACBD at pre-establishedintervals to trigger them to provide a l [.L sec. interrogate pulse.

When the interrogate pulses are coupled to the scanpoints, during theclose test, the ground at the input of the scanpoints will cause thetransistors 68 to conduct and return an AC signal to the AC busreceivers ACBR, via the conductors 69. This AC signal is stored in theAC bus receivers, and coupled to the input NAND gate 70 of the scanpointtest fail detector.

If all of the AC bus receivers ACBR do not receive such an AC signalduring the close test, the flip-flop TEST FAILED will be set, via theAND gate 74 and the OR gate 76. Upon coincidence of the strobe pulse andthe TEST signal from the flip-flop TEST, the flip-flop TEST FAILED willbe triggered to couple an output to an alarm circuit in'the maintenancesubsystem.

If during the open test one or more of the AC bus receivers ACBR detectsor receives an AC signal pulse from the scanpoints, the AC signal againis stored and coupled to the input NAND gate 71, after being inverted bythe inverters 72. The output of the NAND gate 711 is coupled through theAND gate 75 and the OR gate 76 to again set the flip-flop TEST FAILED,which upon coincidence of the strobe pulse and the TEST output of theflip-flop TEST is triggered to sound an alarm.

The AND gates 74 and 75 are enabled by the coincidence'of the output ofthe respective NAND gates 70 and 71 by the output, OT and CT from theflip-flop TEST mode, and the TFE (trunk frame equipped) signal suppliedform the equipment group register. This TFE signal is a zero if theparticular frame is not equipped with scanpoints, and is provided todisable the flip-flop TEST FAILED, to prevent erroneous test results.

The AC bus drivers ACBD are tested only during the scanpoint close test,and the test is provided by the DTT and DUT words stored in the trunkscanner memory (FIG. 2), following the test group words. When the DTTword is read, only the units address is supplied to the AC bus driversACBD. If any of the AC bus receivers ACBR receives an AC signal, it isan indication that the tens address lead in the AC bus driver ACBDfailed, otherwise it could not have supplied an output interrogate pulseto the scanpoint. When the DUT word is read, only the tens address issupplied to the AC bus drivers ACBD, to determine if any of the unitleads in the AC bus drivers have failed. In each case, if an AC signalis received by any of the AC bus receivers ACBR, the flip-flop TESTFAILED is set to cause an alarm to be sounded.

When the end test word (instruction I14) is read from memory (FIG. 2),the scanpoint test circuitry is disabled so that normal scanning canbegin again.

The AC bus receivers ACBR are tested for both set and reset, each timeprior to activating the AC bus drivers to insure their operationalcapabilities before they are to receive AC signals from the scanpoints.As can be seen in FIGS. 2 and 6, whenever a group word (instruction I1)is read from the status section of the trunk scanner memory or a testgroup word (instruction I13) is read from the test section, an ACreceiver test circuit 80 is energized. This circuit 80 first couples aSET pulse to all of the AC bus receivers to set them, which actioncauses an output signal to be coupled to the NAND gate 70. If all of theAC bus receivers ACBR set, an ALL SET signal is coupled from the outputof the NAND gate 70 to the AC receiver test circuit 80. The AC busreceivers ACBR then are reset, by means ofa RESET signal from the ACreceiver test circuit 80, and if all reset, an ALL RESET signal iscoupled from the output of the NAND gate 71 to the AC receiver testscircuit 80. The SET and RESET signals are generated at fixed timeintervals, and if the ALL SET and ALL RESET signals are not received bythe AC receiver test circuit within correspondingly fixed timeintervals, an alarm is triggered to indicate that all of the AC busreceivers ACBR did not set, or reset.

The only portion of the above described status identification circuitrywhich is not specifically tested is the AC bus to the scanpoints.However, its condition is inferred through a process of elimination.That is, if all of the other tests pass, that is, the driver test, theAC bus receiver test and the scanpoint test, then the AC bus itself musthave failed.

From the above description, it can be seen that the testing of thestatus identification circuitry is in effect a total test in that eachof the various individual portions of the circuitry are specificallytested, with only the condition of the AC bus itself being left toinference. However, even in this case, a failure of an AC bus can beimmediately detected, since if all of the other tests passed, thefailure must be in the AC bus. Furthermore, the manner in which thetests are performed is such that the area or portion of the circuitrywhere the failure occurred is indicated so that corrective action can betaken immediately. Further still, by performing the open test and theclose test on alternate scan cycles, the same test functions can be usedfor both tests without additional memory being required.

It will thus be seen that the objects set forth above among those madeapparent from the preceding description, are efficiently attained andcertain changes may be made in carrying out the above method and in theconstruction set forth. Accordingly, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

Now that the invention has been described, what is claimed as new anddesired to be secured by Letters Patent is:

1. In a system including a plurality of trunks each of which has a trunkticketing contact, trunk status identification circuitry for determiningthe open or closed status of said trunk ticketing contacts including anAC bus driver, a scanpoint and an AC bus receiver associated with eachof said trunk ticketing contacts, said AC bus drivers and receiversbeing coupled to said scanpoints by an AC bus, and a memory for storinga plurality of memory words including a status section containingaddress words for addressing said trunk circuits and for storing thestatus thereof and a test section containing address words and testwords for addressing said trunk circuits and for testing the operabilityof said trunk status identification circuitry, said memory words storedin said memory being addressed sequentially during each scan cycle, saidaddress words in said status section upon being read therefrom operatingsaid AC bus drivers to transmit an interrogate pulse to said scanpointsto activate them to provide an AC signal to said AC bus receivers toindicate the open or closed status of said trunk ticketing contacts, anarrangement for detecting failures in said status identificationcircuitry comprising activating means coupled to said scanpoints andoperable to activate them to simulate an open and a closed status ofsaid trunk ticketing contacts and thereby provide an open test" and aclose test thereof, respectively; said address words in said testsection upon being read therefrom operating said AC bus drivers totransmit an interrogate pulse to said scanpoints to activate them toprovide a signal to said AC bus receivers indicating the status of saidtrunk ticketing contacts in accordance with the simulated open or closedstatus thereof; all of said AC bus receivers being coupled to first andsecond gate means; said first gate means providing an output to triggeralarm means during said open test when any one of said AC bus receiversfails to provide a signal to it; and said second gate means providing anoutput to trigger alarm means during said close test when any one ofsaid AC bus receivers fails to provide a signal to it, whereby thecondition of each of said scanpoints can be tested.

2. The arrangement of claim 1, further including test mode means coupledto said activating means, said test mode means being operated when astart test word in said test section is read therefrom to energize saidactivating means to initiate said open test and said close test andbeing operated when an end test word in said test section is readtherefrom to de-energize said activating means to terminate said opentest" and said close test.

3. The arrangement of claim 2, wherein said test mode means isalternately operated during each scan cycle to energize said activatingmeans to initiate one of said open test and said close test, wherebysaid open test and said close test are performed during alternate scancycles.

4. The arrangement of claim 3, wherein said activating means comprisesfirst circuit means normally operable to couple a contact statusidentification potential to said trunk ticketing contacts, secondcircuit means operable during said close test to couple a contact statusidentification potential to said scanpoints to simulate a closed trunkticketing contact status, test driver means coupled to said first andsecond circuit means, said test mode meanswhen said start test word isread energizing said test driver means to de-energize said first circuitmeans to remove said contact status identification potential from saidtrunk ticketing contacts, said test mode means during said close testfurther energizing said second circuit means to couple said contactstatus identification potential to said scanpoints to simulate saidclosed trunk ticketing contact status and during said open testde-energizing said second circuit means to remove said contact statusidentification potential.

5. The arrangement of claim 1, wherein said address words in said testsection for operating said AC bus drivers comprise a first and a secondaddress word, said AC bus drivers being operated upon receipt of saidfirst and second address words in coincidence, said test section furtherincluding a corresponding first address word and a corresponding secondaddress word, each of which is separately read therefrom each scan cycleand transmitted to said AC bus drivers, the receipt of an AC signal byany one of said AC bus receivers when either said corresponding firstaddress word or said corresponding second address word is transmitted tosaid AC bus drivers functioning to trigger said alarm means therebyindicating a failure of an AC bus driver, whereby said AC bus driverscan be separately tested from said scanpoints.

6. The arrangement of claim 5, wherein said AC bus drivers are testedduring said close test.

7. The arrangement of claim ll, further including AC receiver test meansoperated to set and reset each of said AC bus receivers, said AC busreceivers upon being set providing a signal to said first gate means andsaid first gate means providing an ALL SET signal to said AC receivertest means when all of said AC bus receivers set, said AC bus receiversupon being reset providing a signal to said second gate means and saidsecond gate means providing an ALL RESET signal to said AC receiver testmeans when all of said AC bus receivers reset, said AC receiver testmeans activating alarm means when said ALL SET and ALL RESET signals arenot received within an established time interval,

' whereby the'condition of each of said AC bus receivers can be tested.

8. The arrangement of claim 7, wherein said AC receiver test means areoperated to set and reset said AC bus receivers each time an addressword in said status section is readtherefrom.

9. The arrangement of claim 1, wherein said test section of said memoryfurther includes as a part of said address words a trunk equipped wordindicating those trunk circuits equipped with scanpoints, said trunkequipped words being transmitted in coincidence with the output of'saidfirst and'second gate means to trigger said alarm, whereby those trunkcircuits which are not lid equipped with scanpoints do not cause afaulty alarm indication.

10. The arrangement of claim 5, further including AC receiver test meansoperated to set and reset each of said AC bus receivers, said AC busreceivers upon being set providing a signal to said first gate means andsaid first gate means providing an ALL SET signal to said AC receivertest means when all of said AC bus receivers set, said AC bus receiversupon being reset providing a signal to said second gate means and saidsecond gate means providing an ALL RESET signal to said AC receiver testmeans when all of said AC bus receivers reset, said AC receiver testmeans activating alarm means when said ALL SET and ALL RESET signals arenot received within an established time interval, whereby the conditionof each of said AC bus receivers can be tested.

11. A method of detecting failures in the status identificationcircuitry in a system including a plurality of trunks each of which hasa trunk ticketing contact, trunk status identification circuitry fordetermining the open or closed status of said trunk ticketing contactsincluding an AC bus driver, a scanpoint and an AC bus receiverassociated with each of said trunk ticketing contacts, said AC busdrivers and receivers being coupled to said scanpoints by an AC bus, anda memory for storing a plurality of memory words including a statussection containing address words for addressing said trunk circuits andfor storing the status thereof and a test section containing addresswords and test words for addressing said trunk circuits and for testingthe operability of said trunk status identification circuitry, saidmemory words stored in said memory being addressed sequentially duringeach scan cycle, said address words in said status section upon beingread therefrom operating said AC bus drivers to transmit an interrogatepulse to said scanpoints to activate them to provide an AC signal tosaid AC bus receivers to indicate the open or closed status of saidtrunk ticketing contacts, said method comprising the steps of activatingsaid scanpoints to simulate an open and a closed status of said trunkticketing contacts and thereby provide an open test and a close testthereof, respectively; reading said address words in said test sectionand transmitting them to said AC bus drivers to operate them to transmitan interrogate pulse to said scanpoints to activate them to provide asignal to said AC bus receivers indicating the status of said trunkticketing contacts in accordance with the simulated open or closedstatus thereof; coupling all of said AC bus receivers to first andsecond gate means; said first gate means providing an output to triggeralarm means during said open test when any one of said AC bus receiversfails to provide a signa] to it; and said second gate means providing anoutput to trigger alarm means during said close test when any one ofsaid AC bus receivers fails to provide a signal to it, whereby thecondition of each of said scanpoints can be tested.

12. The method of claim 111, further including the steps of activatingsaid scanpoints to perform said open test and said close test onalternate scan cycles.

13 The method of claim 11, further including the steps of settingandresetting said AC bus receivers and providing a signal to activatealarm means when any one of said AC bus'receivers fails to set or reset.

16 to activate said AC bus drivers to them, and activating said alarmmeans if any one of said AC bus receivers receives a signal from saidscanpoints, whereby the operability of said AC bus drivers can betested.

* k I! k

1. In a system including a plurality of trunks each of which has a trunkticketing contact, trunk status identification circuitry for determiningthe open or closed status of said trunk ticketing contacts including anAC bus driver, a scanpoint and an AC bus receiver associated with eachof said trunk ticketing contacts, said AC bus drivers and receiversbeing coupled to said scanpoints by an AC bus, and a memory for storinga plurality of memory words including a status section containingaddress words for addressing said trunk circuits and for storing thestatus thereof and a test section containing address words and testwords for addressing said trunk circuits and for testing the operabilityof said trunk status identification circuitry, said memory words storedin said memory being addressed sequentially during each scan cycle, saidaddress words in said status section upon being read therefrom operatingsaid AC bus drivers to transmit an interrogate pulse to said scanpointsto activate them to provide an AC signal to said AC bus receivers toindicate the open or closed status of said trunk ticketing contacts, anarrangement for detecting failures in said status identificationcircuitry comprising activating means coupled to said scanpoints andoperable to activate them to simulate an open and a closed status ofsaid trunk ticketing contacts and thereby provide an ''''open test''''and a ''''close test'''' thereof, respectively; said address words insaid test section upon being read therefrom operating said AC busdrivers to transmit an interrogate pulse to said scanpoints to activatethem to provide a signal to said AC bus receivers indicating the statusof said trunk ticketing contacts in accordance with the simulated openor closed status thereof; all of said AC bus receivers being coupled tofirst and second gate means; said first gate means providing an outputto trigger alarm means during said ''''open test'''' when any one ofsaid AC bus receivers fails to provide a signal to it; and said secondgate means providing an output to trigger alarm means during said''''close test'''' when any one of said AC bus receivers fails toprovide a signal to it, whereby the condition of each of said scanpointscan be tested.
 2. The arrangement of claim 1, further including testmode means coupled to said activating means, said test mode means beingoperated when a start test word in said test section is read therefromto energize said activating means to initiate said ''''open test'''' andsaid ''''close test'''' and being operated when an end test word in saidtest section is read therefrom to de-energize said activating means toterminate said ''''open test'''' and said ''''close test''''.
 3. Thearrangement of claim 2, wherein said test mode means is alternatelyoperated during each scan cycle to energize said activating means toinitiate one of said ''''open test'''' and said ''''close test'''',whereby said ''''open test'''' and said ''''close test'''' are performedduring alternate scan cycles.
 4. The arrangement of claim 3, whereinsaid activating means comprises first circuit means normally operable tocouple a contact status identification potential to said trunk ticketingcontacts, second circuit means operable during said ''''close test''''to couple a contact status identification potential to said scanpointsto simulate a closed trunk ticketing contact status, test driver meanscoupled to said first and second circuit means, said test mode meanswhen said start test word is read energizing said test driver means tode-energize said first circuit means to remove said contact statusidentification potential from said trunk ticketing contacts, said testmode means during said ''''close test'''' further energizing said secondcircuit means to couple said contact status identification potential tosaid scanpoints to simulate said closed trunk ticketing contact statusand during said ''''open test'''' de-energizing said second circuitmeans to remove said contact status identification potential.
 5. Thearrangement of claim 1, wherein said address words in said test sectionfor operating said AC bus drivers comprise a first and a second addressword, said AC bus drivers being operated upon receipt of said first andsecond address words in coincidence, said test section further includinga corresponding first address word and a corresponding second addressword, each of which is separately read therefrom each scan cycle andtransmitted to said AC bus drivers, the receipt of an AC signal by anyone of said AC bus receivers when either said corresponding firstaddress word or said corresponding second address word is transmitted tosaid AC bus drivers functioning to trigger said alarm means therebyindicating a failure of an AC bus driver, whereby said AC bus driverscan be separately tested from said scanpoints.
 6. The arrangement ofclaim 5, wherein said AC bus drivers are tested during said ''''closetest''''.
 7. The arrangement of claim 1, further including AC receivertest means operated to set and reset each of said AC bus receivers, saidAC bus receivers upon being set providing a signal to said first gatemeans and said first gate means providing an ALL SET signal to said ACreceiver test means when all of said AC bus receivers set, said AC busreceivers upon being reset providing a signal to said second gate meansand said second gate means providing an ALL RESET signal to said ACreceiver test means when all of said AC bus receivers reset, said ACreceiver test means activating alarm means when said ALL SET and ALLRESET signals are not received within an established time interval,whereby the condition of each of said AC bus receivers can be tested. 8.The arrangement of claim 7, wherein said AC receiver test means areoperated to set and reset said AC bus receivers each time an addressword in said status section is read therefrom.
 9. The arrangement ofclaim 1, wherein said test section of said memory further includes as apart of said address words a trunk equipped word indicating those trunkcircuits equipped with scanpoints, said trunk equipped words beingtransmitted in coincidence with the output of said first and second gatemeans to trigger said alarm, whereby those trunk circuits which are notequipped with scanpoints do not cause a faulty alarm indication.
 10. Thearrangement of claim 5, further including AC receiver test meansoperated to set and reset each of said AC bus receivers, said AC busreceivers upon being set providing a signal to said first gate means andsaid first gate means providing an ALL SET signal to said AC receivertest means when all of said AC bus receivers set, said AC bus receiversupon being reset providing a signal to said second gate means and saidsecond gate means providing an ALL RESET signal tO said AC receiver testmeans when all of said AC bus receivers reset, said AC receiver testmeans activating alarm means when said ALL SET and ALL RESET signals arenot received within an established time interval, whereby the conditionof each of said AC bus receivers can be tested.
 11. A method ofdetecting failures in the status identification circuitry in a systemincluding a plurality of trunks each of which has a trunk ticketingcontact, trunk status identification circuitry for determining the openor closed status of said trunk ticketing contacts including an AC busdriver, a scanpoint and an AC bus receiver associated with each of saidtrunk ticketing contacts, said AC bus drivers and receivers beingcoupled to said scanpoints by an AC bus, and a memory for storing aplurality of memory words including a status section containing addresswords for addressing said trunk circuits and for storing the statusthereof and a test section containing address words and test words foraddressing said trunk circuits and for testing the operability of saidtrunk status identification circuitry, said memory words stored in saidmemory being addressed sequentially during each scan cycle, said addresswords in said status section upon being read therefrom operating said ACbus drivers to transmit an interrogate pulse to said scanpoints toactivate them to provide an AC signal to said AC bus receivers toindicate the open or closed status of said trunk ticketing contacts,said method comprising the steps of activating said scanpoints tosimulate an open and a closed status of said trunk ticketing contactsand thereby provide an ''''open test'''' and a ''''close test''''thereof, respectively; reading said address words in said test sectionand transmitting them to said AC bus drivers to operate them to transmitan interrogate pulse to said scanpoints to activate them to provide asignal to said AC bus receivers indicating the status of said trunkticketing contacts in accordance with the simulated open or closedstatus thereof; coupling all of said AC bus receivers to first andsecond gate means; said first gate means providing an output to triggeralarm means during said ''''open test'''' when any one of said AC busreceivers fails to provide a signal to it; and said second gate meansproviding an output to trigger alarm means during said ''''closetest'''' when any one of said AC bus receivers fails to provide a signalto it, whereby the condition of each of said scanpoints can be tested.12. The method of claim 11, further including the steps of activatingsaid scanpoints to perform said ''''open test'''' and said ''''closetest'''' on alternate scan cycles.
 13. The method of claim 11, furtherincluding the steps of setting and resetting said AC bus receivers andproviding a signal to activate alarm means when any one of said AC busreceivers fails to set or reset.
 14. The method of claim 13, whereinsaid AC bus receivers are set and reset each time an address word insaid status section is read from memory.
 15. The method of claim 13,further including the step of transmitting a signal corresponding to aportion of the address word in said test section normally used toactivate said AC bus drivers to them, and activating said alarm means ifany one of said AC bus receivers receives a signal from said scanpoints,whereby the operability of said AC bus drivers can be tested.